This invention relates to improved materials usable as electrode active materials, and electrodes formed from it for electrochemical cells in batteries.
Lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between spaced apart positive and negative electrodes. Batteries with anodes of metallic lithium and containing metal chalcogenide cathode active material are known. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. Other electrolytes are solid electrolytes typically called polymeric matrixes that contain an ionic conductive medium, typically a metallic powder or salt, in combination with a polymer that itself may be ionically conductive which is electrically insulating. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. Cells having a metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemically active (electroactive) material of the cathode whereupon they release electrical energy to an external circuit.
It has recently been suggested to replace the lithium metal anode with an insertion anode, such as a lithium metal chalcogenide or lithium metal oxide. Carbon anodes, such as coke and graphite, are also insertion materials. Such negative electrodes are used with lithium- containing insertion cathodes, in order to form an electroactive couple in a cell. Such cells, in an initial condition, are not charged. In order to be used to deliver electrochemical energy, such cells must be charged in order to transfer lithium to the anode from the lithium- containing cathode. During discharge the lithium is transferred from the anode back to the cathode. During a subsequent recharge, the lithium is transferred back to the anode where it reinserts. Upon subsequent charge and discharge, the lithium ions (Li+) are transported between the electrodes. Such rechargeable batteries, having no free metallic species are called rechargeable ion batteries or rocking chair batteries. See U.S. Pat. Nos. 5,418,090; 4,464,447; 4,194,062; and 5,130,211.
Preferred positive electrode active materials include LicoO2, LiMn2O4, and LiNiO2. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn2O4, for which methods of synthesis are known. The lithium cobalt oxide (LiCoO2), the lithium manganese oxide (LiMn2O4), and the lithium nickel oxide (LiNiO2) all have a common disadvantage in that the charge capacity of a cell comprising such cathodes suffers a significant loss in capacity. That is, the initial capacity available (amp hours/gram) from LiMn2O4, LiNiO2, and LicoO2 is less than the theoretical capacity because significantly less than 1 atomic unit of lithium engages in the electrochemical reaction. Such an initial capacity value is significantly diminished during the first cycle operation and such capacity further diminishes on every successive cycle of operation. For LiNiO2 and LiCoO2 only about 0.5 atomic units of lithium is reversibly cycled during cell operation. Many attempts have been made to reduce capacity fading, for example, as described in U.S. Pat. No. 4,828,834 by Nagaura et al. However, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing electrode material having acceptable capacity without disadvantage of significant capacity loss when used in a cell.
The invention provides novel lithium-metal-fluorophosphate materials which, upon electrochemical interaction, release lithium ions, and are capable of reversibly cycling lithium ions. The invention provides a rechargeable lithium battery which comprises an electrode formed from the novel lithium-metal-fluorophosphates. Methods for making the novel lithium-metal-fluorophosphates and methods for using such lithium-metal-fluorophosphates in electrochemical cells are also provided. Accordingly, the invention provides a rechargeable lithium battery which comprises an electrolyte; a first electrode having a compatible active material; and a second electrode comprising the novel lithium-metal-fluorophosphate materials. The novel materials, preferably used as a positive electrode active material, reversibly cycle lithium ions with the compatible negative electrode active material. Desirably, the lithium-metal-fluorophosphate is represented by the nominal general formula LiM1xe2x88x92yMIyPO4F where 0xe2x89xa6yxe2x89xa61. Such compounds include LiMPO4F for y=0. Such compounds are also represented by Li1xe2x88x92xMPO4F and Li1xe2x88x92xM1xe2x88x92yMIyPO4F, where in an initial condition, xe2x80x9cxxe2x80x9d is essentially zero; and during cycling a quantity of xe2x80x9cxxe2x80x9d lithium is released where 0xe2x89xa6xxe2x89xa61. Correspondingly, M has more than one oxidation state in the lithium-metal-fluorophosphate compound, and more than one oxidation state above the ground state M0. The term oxidation state and valence state are used in the art interchangeably. Also, MI may have more than one oxidation state, and more than one oxidation state above the ground state MIxc2x0.
Desirably, M is selected from V (vanadium), Cr (chromium), Fe (iron), Ti (titanium), Mn (manganese), Co (cobalt), Ni (nickel), Nb (niobium), Mo (molybdenum), Ru (ruthenium), Rh (rhodium) and mixtures thereof. Preferably, M is selected from the group V, Cr, Fe, Ti, Mn, Co, and Ni. As can be seen, M is preferably selected from the first row of transition metals, and M preferably initially has a +3 oxidation state. In another preferred aspect, M is a metal having a +3 oxidation state and having more than one oxidation state, and is oxidizable from its oxidation state in lithium-metal-fluorophosphate compound. In another aspect, MI is a metal having a +3 oxidation state, and desirably MI is an element selected from the group V, Cr, Fe, Ti, Mn, Co, Ni, Nb, Mo, Ru, Rh, B (boron) and Al (aluminum).
In a preferred aspect, the product LiM1xe2x88x92yMIyPO4F is a triclinic structure. In another aspect, the xe2x80x9cnominal general formulaxe2x80x9d refers to the fact that the relative proportions of the atomic species may vary slightly on the order of up to 5 percent, or more typically, 1 percent to 3 percent. In another aspect the term xe2x80x9cgeneralxe2x80x9d refers to the family of compounds with M, MI, and y representing variations therein. The expressions y and 1xe2x88x92y signify that the relative amount of M and MI may vary and that 0xe2x89xa6yxe2x89xa61. In addition, M may be a mixture of metals meeting the earlier stated criteria for M. In addition, MI may be a mixture of elements meeting the earlier stated criteria for MI.
The active material of the counter electrode is any material compatible with the lithium-metal-fluorophosphate of the invention. Where the lithium-metal-fluorophosphate is used as a positive electrode active material, metallic lithium may be used as the negative electrode active material where lithium is removed and added to the metallic negative electrode during use of the cell. The negative electrode is desirably a nonmetallic insertion compound. Desirably, the negative electrode comprises an active material from the group consisting of metal oxide, particularly transition metal oxide, metal chalcogenide, carbon, graphite, and mixtures thereof. It is preferred that the anode active material comprises a carbonaceous material such as graphite. The lithium-metal-fluorophosphate of the invention may also be used as a negative electrode material.
The starting (precursor) materials include a lithium containing compound, and a metal phosphate compound. Preferably, the lithium containing compound is in particle form, and an example is lithium salt. A particular example of a lithium salt is lithium fluoride (LiF). Preferably, the metal phosphate compound is in particle form, and examples include metal phosphate salt, such as FePO4 and CrPO4. The lithium compound and the metal phosphate compound are mixed in a proportion which provides the stated general formula.
In one aspect, the starting materials are intimately mixed and then reacted together where the reaction is initiated by heat. The mixed powders are pressed into a pellet. The pellet is then heated to an elevated temperature. This reaction can be run under an air atmosphere, or can be run under a non-oxidizing atmosphere. The precursors are commercially available, and include, for example, a lithium fluoride salt, and metal phosphate, such as CrPO4, FePO4, or MnPO4.
In another aspect, the metal phosphate salt used as a precursor for the lithium metal phosphate reaction can be formed either by a carbothermal reaction, or by a hydrogen reduction reaction. Preferably, the phosphate-containing anion compound is in particle form, and examples include metal phosphate salt, diammonium hydrogen phosphate (DAHP), and ammonium dihydrogen phosphate (ADHP). The metal compound for making the precursor are typically metal oxides. In the carbo-thermal reaction, the starting materials are mixed together with carbon, which is included in an amount sufficient to reduce the metal oxide to metal phosphate. The starting materials for the formation of the metal phosphates are generally crystals, granules, and powders and are generally referred to as being in particle form. Although many types of phosphate salts are known, it is preferred to use diammonium hydrogen phosphate (DAHP), or ammonium dihydrogen phosphate (ADHP). Both DAHP and ADHP meet the preferred criteria that the starting materials decompose to liberate the phosphate anion which may then react with the metal oxide compound. Exemplary metal compounds are Fe2O3, Fe3O4, V2O5, VO2, MnO2, Mn2O3, TiO2, Ti2O3, Cr2O3, CoO, Ni3(PO4)2, Nb2O5, Mo2O3, V2O3, FeO, Co3O4, CrO3, Nb2O3, MoO3. The starting materials are available from a number of sources. For example, the metal oxides, such as vanadium pentoxide or iron oxide, are available from suppliers including Kerr McGee, Johnson Matthey, or Alpha Products of Davers, Mass.
Objects, features, and advantages of the invention include an electrochemical cell or battery based on lithium-metal-fluorophosphates. Another object is to provide a cathode active material which combines the advantages of good discharge capacity and capacity retention. It is also an object of the present invention to provide positive electrodes which can be manufactured economically. Another object is to provide a cathode active material which can be rapidly and cheaply produced and lends itself to commercial scale production for preparation of large quantities.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.